The present invention relates to a pigment wherein the color depends on the viewing angle, obtained by three dimensional crosslinking of oriented substances of liquid-crystalline structure having a chiral phase.
The present invention additionally relates to a process for the preparation of a pigment wherein the color depends on the viewing angle, in which oriented substances of liquid-crystalline structure having a chiral phase are three-dimensionally crosslinked.
The invention further relates to a coating material which comprises a resin binder and such a pigment and to a motor vehicle finished with such a coating material.
A pigment of this kind and its preparation and use in a coating material, in particular in a finish for motor vehicles, is known from Mueller-Rees et al., U.S. Pat. No. 5,362,315 (counterpart of German Patent DE 42 40 743), the entire disclosure of which is herein incorporated by reference.
Liquid-crystalline substances used for the preparation of such pigments have a twisted structure whose pitch corresponds to the wavelength of light in the region of UV to IR. This structure is found, for example, in cholesteric liquid crystals. Cholesteric liquid crystals or in general liquid-crystalline substances with a chiral phase and a twisted structure with a desired pitch can be obtained from nematic, smectic or discotic structures by adding a chiral substance to them. The nature and quantity of the chiral substance determine the pitch of the twisted structure and therefore the wavelength of the reflected light. The structure can be twisted either left-handed or right-handed. The starting substances, moreover, have groups which are available for polymerization, either by condensation polymerization or addition polymerization.
These substances become oriented, for example during application with a doctor blade to a film. Subsequently, the substances are crosslinked, for example by irradiation with UV light. This gives rise to the pigment, which after detachment from the film, can be brought to the desired particle size.
These pigments can be incorporated into coating materials or coating systems, especially coating materials for the finishing of motor vehicles. A motor vehicle finished in this way exhibits a color, such as blue, green or red shades, which differs at a single point depending on the viewing angle.
In the practical use of such pigments in the finishing of motor vehicles, it has now been found that the color changes are dependant to a great extent on the baking or drying temperature of the coating material.
Although it is desirable that such color pigments should produce a color which differs depending on the viewing angle, it is not desirable for the shades which result from a specific viewing angle to be different depending on the baking or drying temperature or on the temperature treatments of the pigments. This is not only undesirable when using the pigments in coating materials, especially for motor vehicle finishes, but for all applications of the pigments in which they are subjected to heat treatments. For instance, another area in which such pigments can be used is the cosmetics industry, in which creams, pastes or the like are heated, for example, for sterilization. In these areas of application too, no temperature-dependent color shift should occur.
When the pigments are used in coating systems for finishing motor vehicles, the coating materials of the original finish of the bodywork are baked or dried at about 130.degree. C. In subsequent refinishing operations, however, it is not possible to use such high temperatures, because other components of the motor vehicle can be damaged. Refinishing operations are therefore carried out at about 80.degree. C. In practice, it has been found that these different baking or drying temperatures induce, in a single given pigment, severe shifts in color, especially blue shifts.
An object of the present invention is therefore to remedy this situation and to continue developing pigments in which the color from a particular viewing angle remains largely unaltered by the action of elevated temperatures.
In the case of a pigment, the object is achieved according to the present invention by crosslinking in the presence of at least one color-neutral compound comprising at least two crosslinkable double bonds.
In the case of a process, the object is achieved by admixing the substances prior to crosslinking with at least one color-neutral compound comprising at least two crosslinkable double bonds.
By admixing crosslinkable compounds which comprise at least two double bonds it is possible to increase the crosslinking density of the pigments. At the same time a certain dilution of reactive components which constitute the color pigments is achieved, and these "reactive diluents" are integrated, in the course of crosslinking, into the resulting three-dimensional matrix.
Surprisingly it has been found that by diluting on the one hand and by increasing the crosslinking density on the other hand, the color of the resulting pigments per se is not altered, and at the same time, a high color stability at elevated temperatures is achieved, thus reducing the color shift.
"Color-neutral" compounds in the context of the present invention are those which do not exert any unwanted effect on the resulting color of the reactive substances of liquid-crystalline structure having a chiral phase, and whose functionality creates the possibility of additional crosslinking points.
In a preferred embodiment, the color-neutral compounds have more than three cross linkable double bonds. This increased functionality greatly increases the crosslinking densities of the pigments according to the invention and particularly contributes to color stability at elevated temperatures.
In a further preferred embodiment, the color-neutral compounds have double bonds of the acrylate, methacrylate, vinyl and/or allyl type.
These measures are advantageous in that these types of double bonds are similar to those functional groups which provide for three-dimensional crosslinking in the substances of liquid-crystalline structure having a chiral phase, so that the compounds which now have additionally been admixed can be crosslinked using the same initiators as the actual substances constituting the pigments. If, for example, the substances of the liquid-crystalline structure selected are those which can be crosslinked with UV light, then corresponding, UV-crosslinkable color neutral compounds can be added in order to achieve the desired color stability effect. Accordingly, this is done by mixing in the corresponding compounds prior to crosslinking, without needing to employ additional or different initiators.
In a further preferred embodiment, color-neutral compounds are selected from the group consisting of acrylates, polyurethanes, epoxides, siloxanes, polyesters and alkyd resins.
In a further preferred embodiment, at least one color-neutral compound is selected from the group consisting of 1,6-hexanediol diacrylate, divinylbenzene, trivinylalkoxysilanes, trifunctional polyurethane acrylate oligomers, mixtures of pentaerythritol tri- and tetraacrylates, tetrafunctional acrylate oligomers, tetraallylsilane, tetravinylsilane, tetrakis(2-methacryloyloxyalkoxy)silanes, 1,3,5,7-tetravinyltetraalkylcyclotetrasiloxanes, hexafunctional polyurethane acrylate oligomers and dipentaerythritol hexaacrylate.
In a further preferred embodiment, the content of the color-neutral compound is from 1 to 20% by weight, preferably from 5 to 15% by weight, based on the total weight of the overall mixture to be crosslinked.
The pigments can be incorporated into conventional coating systems, in which case the pigments are ground and, if desired, screened to a suitable particle size.
If a motor vehicle is finished with such a coating material or coating system, the color shift observed is greatly reduced relative to the corresponding pigments without additional color-neutral compounds, even in the case of large differences in baking or drying temperature. Thus, for example, the .DELTA.E value of coating materials provided with color pigments of the type specified at the outset can be reduced by more than 70% if the colors of coated samples are compared at baking or drying temperatures of room temperature and 130.degree. C. Correspondingly pronounced results are obtained if the colors of coated panels are compared at baking or drying temperatures of 80.degree. C. and 130.degree. C. Similar comments apply to the .DELTA..lambda. values.
It is understood that the features mentioned above and below can be employed not only in the combinations indicated but also individually or in different combinations within the scope of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention.